Lubricant for thermoplastic resin which is a three stage polymer

ABSTRACT

A lubricant for a thermoplastic resin, which is a three-stage polymer obtained by (A) polymerizing 80-100 w.t.% of methyl methacrylate and 0-20 w.t.% of other copolymerizable monomer so that the reduced viscosity ηsp/C of the obtained polymer is not higher than 2 dl/g, (B) polymerizing 30-70 weight parts of a monomer mixture composed of 20-80 w.t.% of an acrylic acid ester and 80-20 w.t.% of styrene or a substituted styrene in the presence of 10-60 weight parts of the polymer obtained at stage (A), so that the reduced viscosity ηsp/C of the copolymer obtained by polymerizing 5 to 30 weight parts of a monomer or monomer mixture composed of 50-100 w.t.% of a methacrylic acid ester and 0-50 w.t.% of other copolymerizable monomer in the presence of the two-stage polymer formed through stages (A) and (B), is that the total amount of the components used at staged (A), (B) and (C) is 100 weight parts and the reduced viscosity ηsp/C of the polymer obtained by polymerizing the monomer or monomer mixture in the absence of the two-stage polymer formed through stages (A) and (B) is not higher than 1.5 dl/g.

This application is a divisional of copending application Ser. No.08/013,986, filed Feb. 5, 1993 now U.S. Pat. No. 5,314,965 which is acontinuation of U.S. Pat. No. 07/426,187 filed Oct. 25, 1989 nowabandoned.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a lubricant for a thermoplastic resinand a thermoplastic resin composition comprising this lubricant. Moreparticularly, the present invention relates to a lubricant havingexcellent properties such that, when the lubricant is incorporated in athermoplastic resin such as a vinyl chloride resin, a polycarbonateresin, a polyester resin, an ABS resin, a styrene resin, a methacrylicresin or a polyethylene resin, a thermoplastic resin composition showingexcellent flowability and release property at the molding step andproviding a molded article having a good gloss and transparency isobtained without an adverse affect on the characteristics inherentlypossessed by the thermoplastic resin.

(2) Description of the Related Art

Thermoplastic resins, especially vinyl chloride resins, have excellentphysical and chemical properties and thus are widely used.

Nevertheless, vinyl chloride resins have a problem in that theprocessability in a broad sense is poor, and although plasticizer, aprocessing aid composed mainly of methyl methacrylate, a lubricant orthe like has been used for solving this problem, a general solutioncannot be obtained by this method.

As means for solving this problem, improved vinyl chloride resincompositions have been proposed in U.S. Pat. No. 3,925,510.

In the proposed vinyl chloride resin compositions, maintenance of thetransparency possessed by the vinyl chloride resin and improvement ofthe processability such as the drawdown effect, the flowability at themolding step, the elongation at a high temperature, the release propertyfrom the roll surface at the calendering step, and the long-perioddurability of lubricating properties are intended, and certain effectsare attained.

Nevertheless, in view of the improvement of the productivity andquality, and to obtain an energy-saving effect, it is desirable todevelop a vinyl chloride resin having a high durability of lubricatingproperties at the processing step and capable of providing a moldedarticle having a good gloss and transparency, and the proposedcompositions were still unsatisfactory. Moreover, an increase ofsubstances adhering to the mold and the like (plate-out) occurs undercertain molding conditions, and the proposed compositions cannotcompletely satisfy the market requirements.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-mentionedproblem, that is, to provide a lubricant for a thermoplastic resin,which shows an excellent flowability and release property at the moldingstep and provides molded article having a good gloss and transparency,and a thermoplastic resin composition comprising this lubricant.

More specifically, in accordance with the present invention, there isprovided a lubricant for a thermoplastic resin, which is a three-stagepolymer obtained by the following sequential these-stage polymerizationprocess. In the first stage, 80 to 100% by weight of methyl methacrylateand 0 to 20% by weight of other monomer copolymerizable therewith arepolymerized so that the reduced viscosity ηsp/C (as determined at 25° C.with respect to a solution of 0.1 g of the polymer in 100 ml ofchloroform) of the obtained polymer is not higher than 2 dl/g.

In the second stage, 30 to 70 parts by weight of a monomer mixturecomprising 20 to 80% by weight of an acrylic acid ester and 80 to 20% byweight of styrene or a substituted styrene is polymerized in thepresence of 10 to 60 parts by weight of the polymer obtained at thefirst stage, so that the reduced viscosity ηsp/C of the copolymerobtained by polymerizing the monomer mixture in the absence of thepolymer obtained at the first stage is not higher is than 1 dl/g.

In the third stage, 5 to 30 parts by weight of a monomer or monomermixture comprising 50 to 100% by weight of a methacrylic acid ester and0 to 50% by weight of other monomer copolymerizable therewith ispolymerized in the presence of the two-stage polymer formed through thefirst and second stages, so that the total amount of the components usedat the three-stages is 100 parts by weight and the reduced viscosityηsp/C of the polymer obtained by polymerizing the monomer or monomermixture in the absence of the two-stage polymer formed through the firstand second stages is not higher than 1.5 dl/g.

Furthermore, according to the present invention, there is provided athermoplastic resin composition comprising 100 parts by weight of athermoplastic resin and 0.05 to 10 parts by weight of theabove-mentioned lubricant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The thermoplastic resin in which the lubricant of the present inventionis incorporated includes a vinyl chloride resin, a polycarbonate resin,a polyolefin resin, a polyester resin, a vinyl fluoride resin, an ABSresin, a styrene resin, and a methacrylic resin. As the vinyl chlorideresin, there can be used a homopolymer of vinyl chloride and a copolymercomprising at least 80% by weight of vinyl chloride units. As themonomer to be copolymerized with vinyl chloride, there can be mentioned,for example, vinyl acetate, ethylene, propylene, styrene, vinyl bromide,vinylidene chloride, acrylic acid esters, and methacrylic acid esters.

The lubricant for a thermoplastic resin according to the presentinvention is a three-stage polymer obtained by a sequential three-stagepolymerization process. This polymer has a sandwich structure in which amethyl methacrylate copolymer and a methacrylic acid ester polymer arearranged on the inner side and outer side, respectively, of a componentimparting lubricating properties, to improve the kneadability andcompatibility with a thermoplastic resin such as a vinyl chloride resin.

Component (A) i.e., the first stage polymer is a homopolymer of methylmethacrylate or a copolymer comprising at least 80% by weight of methylmethacrylate and up to 20% by weight of other monomer copolymerizabletherewith. The monomer to be copolymerized with methyl methacrylate isnot particularly limited, and an appropriate monomer is selectedaccording to the intended object. For example, there can be mentionedmonoethylenically unsaturated monomers such as unsaturated nitriles,vinyl esters, acrylic acid esters, and methacrylic acid esters otherthan methyl methacrylate. These monomers can be used alone or in theform of mixture of two or more thereof. If the amount of the comonomerexceeds 20% by weight, the synergistic effect by the methyl methacrylatepolymer as component (A) and the low-molecular weight polymer ascomponent (B) (i.e., the second stage polymer), for example, thedurability of lubricating properties, is drastically reduced.

To obtain the intended lubricant, a chain transfer agent such as n-octylmercaptan must be used in addition to the above-mentioned monomer andpolymerization must be carried out so that the reduced viscosity ηsp/Cis not higher than 2 dl/g. The reduced viscosity ηsp/C referred toherein is the value measured at 25° C. with respect to a solution of 0.1g of the polymer in 100 ml of chloroform (the same will applyhereinafter). If the reduced viscosity ηsp/C of component (A) exceeds 2dl/g, the processability-improving effect is too high and thelubricating effect is reduced.

The amount of component (A) incorporated is 10 to 60 parts by weight,preferably 10 to 50 parts by 10 weight, per 100 parts by weight of thetotal amount of components (A), (B) and (C), i.e., the total amount ofcomponent (A) exceeds 60 parts by weight, the lubricating effect isreduced, and if the amount of component (A) is smaller than 10 parts byweight, the processability is poor and the durability of the lubricatingproperties is reduced.

Component (B), i.e., the second stage polymer, is a copolymer derivedfrom a monomer mixture comprising 20 to 80% by weight of a monomerselected from acrylic acid esters and 80 to 20% by weight of styrene ora substituted styrene. As the acrylic acid ester, there can bementioned, for example, ethyl acrylate, butyl acrylate, benzyl acrylate,2-ethylhexyl acrylate, cyclohexyl acrylate and phenyl acrylate. Ifstyrene is used for component (B), a composition having an excellenttransparency is obtained, but a substituted styrene such as α-ethylstyrene can be used according to need.

If the amount of the acrylic acid ester is smaller than 20% by weight,the transparency is poor, and if the amount of the acrylic acid esterexceeds 80% by weight, the lubricating effect is reduced.

To impart an especially good lubricating effect to the composition ofthe present invention, use of monomer or monomer mixture giving apolymer having a low glass transition temperature as the acrylics acidester for component (B) is preferred. To obtain an excellent lubricatingeffect, a chain transfer agent such as n-octylmercaptane must be used toreduce the molecular weight, and the polymerization must be carried outso that the reduced viscosity ηsp/C of the copolymer obtained bypolymerizing alone the monomers for component (B) is not higher than 1dl/g. Practically, the polymerization is carried out by appropriatelyadjusting the amounts of the used chain transfer agent and catalyst andthe polymerization temperature.

The amount of component (B) incorporated is 30 to 70 parts by weight,preferably 35 to 70 parts by weight, per 100 parts by weight of thetotal amount of components (A), (B) and (C). If the amount of component(B) is smaller than 30 parts by weight, the lubricating properties ofthe composition are reduced, and if the amount of component (B) islarger than 70 parts by weight, the surface characteristics,transparency and gloss are poor.

Component (C), i.e., the third stage polymer, is obtained bypolymerizing a monomer or monomer mixture comprising 50 to 100% byweight of a methacrylic acid ester and 0 to 50% by weight of othermonomer copolymerizable therewith. As the methacrylic acid ester, therecan be mentioned methyl methacrylate, ethyl methacrylate, butylmethacrylate, benzyl methacrylate, 2-ethylhexyl methacrylate, cyclohexylmethacrylate, and phenyl methacrylate, and methyl methacrylate isespecially preferable form the viewpoint of the compatibility with avinyl chloride resin. The monomer to be copolymerized with themethacrylic acid ester includes ethylenically unsaturated monomers suchas unsaturated nitriles, vinyl esters, and acrylic acid esters. Theamount of component (C) is 5 to 30 parts by weight, preferably 5 to 25parts by weight, per 100 parts by weight of the total amount ofcomponents (A), (B) and (C). If the amount of component (C) is smallerthan 5 parts by weight, secondary agglomeration of the polymer is oftencaused at post treatments such as dehydration and drying, resulting in areduction of the productivity. If the amount of component (C) is largerthan 30 parts by weight, the durability of lubricating properties isreduced.

The reduced viscosity ηsp/C of component (C) can be optionally adjustedby customary procedures by controlling the chain transfer agent, thepolymerization temperature and the like. To obtain the intendedlubricant, the polymerization must be carried out so that the reducedviscosity ηsp/C of component (C) is not higher than 1.5 dl/g, preferablylower than 1.0 dl/g.

The lubricant for a thermoplastic resin comprising the above-mentionedcomponents (A), (B) and (C) is a three-stage polymer, and to obtain arequired lubricating effect, the molecular weight of the three-stagepolymer must be reduced. If high-molecular-weight fractions are presentin components (A) and (C), processing aid effects are manifested. Forexample, at the extrusion molding step, the viscosity of the resin isincreased during the molding, thermal discoloration is conspicuous, andthe lubricating effect is drastically reduced. Therefore, the presenceof such high-molecular-weight fractions is not preferred. The reducedviscosity ηsp/C of the three-stage polymer is not higher than 1.5 dl/gand preferably not higher than 1.0 dl/g.

The three-stage structure of the lubricant for a thermoplastic resinaccording to the present invention can be easily obtained bypolymerizing components (A), (B), and (C) in sequence. As thepolymerization process, there can be adopted, for example, emulsionpolymerization, suspension polymerization and bulk polymerization,emulsion polymerization being preferable. In the process of preparingthe three-stage polymer by emulsion polymerization, to form the intendedthree-layer structure,preferably fresh emulsifier is not added duringthe polymerization at the second and subsequent stages, to substantiallycontrol formation of homopolymers of components (B) and (C). Any knownemulsifiers can be used. For example, ordinary anionic, cationic andnonionic surface active agents can be used. If the pH value of thepolymerization mixture is shifted to the alkaline side according to thekind of emulsifier used, an appropriate pH-adjusting agent can be usedfor preventing the hydrolysis of the acrylic acid ester. As thepH-adjusting agent, there can be used, for example, boric acid/potassiumchloride/sodium hydroxide, potassium dihydrogenphosphate/disodiumhydrogenphosphate, boric acid/potassium chloride/sodium carbonate, boricacid/sodium carbonate, potassium hydrogencitrate/citiric acid, potassiumdihydrogenhosphate/borax, and disodium hydrogenphosphate/citric acid.Water-soluble or oil-soluble, single or redox polymerization initiatorscan be used. For example, water-soluble inorganic initiators such asusual persulfates can be used above or in combination with sulfites,hydrogensulfites or thiosulfates as redox initiators. Moreover, redoxinitiators such as organic peroxide/sodium formaldehyde-sulfoxylate, andazo compounds can be used.

In the process for the preparation of the three-stage polymer, to impartan excellent transparency and good durability of lubricating propertiesto the composition of the present invention, and the molded articleprepared therefrom, the molecular weight of second stage polymer ascomponent (B) must be maintained at a very low level. Namely, thereduced viscosity ηsp/C of the polymer obtained by singly polymerizingthe monomer for component (B) must be not higher than 1 dl/g, preferably0.1 to 0.8 dl/g. If the reduced viscosity is higher than 1, thelubricating effect is reduced and good results can not be obtained. Thereduced viscosity ηsp/C of the third stage polymer as component (C) canbe optionally adjusted by controlling the chain transfer agent and thepolymerization temperature by customary procedures. To obtain theintended lubricant, the polymerization must be carried out so that thereduced viscosity ηsp/C of component (C) is not higher than 1.5 dl/g,preferably not higher than 1.0 dl/g.

In the thermoplastic resin composition of the present invention, thelubricant is incorporate in an amount of 0.05 to 10 parts by weight into100 parts by weight of the thermoplastic resin. If the amount of thelubricant is smaller than 0.05 part by weight a lubricating effectcannot be obtained and the flowability and release property are reduced.If the amount of the lubricant is larger than 10 parts by weight theflowability and release property are higher than desirable andtherefore, a kneading effect is not obtained and surface defects easilyappear on the obtained molded article.

The thermoplastic resin composition of the present invention can beprepared by mixing the thermoplastic resin with the lubricant bycustomary procedures.

Stabilizers such as organic tin compounds, metal soaps such as tin type,barium type and zinc type metal soaps, and epoxy compounds: lubricantssuch as stearic acid, ester waxes, paraffin wax and stearyl alcohol;plasticizers such as phthalic acid esters, phosphoric acid esters, fattyacid esters and epoxy type plasticizers: impact resistance improverssuch as MBS and ABS; colorants such as carbon black and titanium oxide;fillers such as calcium carbonate and asbestos; inorganic foaming agentssuch as ammonium carbonate and sodium bicarbonate; and organic foamingagents such as nitro type foaming agents, sulfohydrazide type foamingagents and azo foaming agents, sulfohydraxide type foaming agents andazo type foaming agent can be incorporated into the thermoplastic resincomposition of the present invention according to need.

The present invention will now be described in detail with reference tothe following examples and comparative examples. Note, all of "parts" inthe examples are by weight.

The reduced viscosities ηsp/C of the polymers of the respective layersof the three-stage polymer of the present invention are the values ofthe polymers obtained by polymerizing alone the monomers for components(A), (B), and (C), respectively, i.e., in the absence of thepreceding-stage polymer. The reduced viscosity ηsp/C of each polymer wasdetermined by the following method.

The monomers for each component were polymerized alone by using apredetermined emulsifier and polymerization initiator at a predeterminedpolymerization temperature while changing the amount of a chain transferagent, and a calibration curve of the reduced viscosity ηsp/C wasprepared and the reduced viscosity ηsp/C of each component was read fromthe calibration curve.

PREPARATION OF LUBRICANTS FOR THERMOPLASTIC RESINS Example 1

A reaction vessel equipped with a stirrer and a reflux cooler wascharged with 280 parts of deionized water, 1.5 parts of sodiumdioctylsulfosuccinate, 2 parts of ammonium persulfate, 0.1 part ofn-octylmercaptan, and 30 parts of methyl methacrylate as the monomer forcomponent (A), and the inner atmosphere of the reaction vessel wasreplaced by nitrogen. The temperature in the reaction vessel waselevated to 65° C. with stirring, and the mixture was stirred at thistemperature for 2 hours. Then a mixture comprising 36 parts of styrene,24 parts of ethyl acrylate and 0.5 part of n-octylmercaptane ofcomponent (B) was added dropwise to the reaction mixture over a periodof 1 hour, and after the dropwise addition, a mixture comprising 10parts of methyl methacrylate and 0.05 part of n-octylmercaptan forcomponent (C) was added to the reaction mixture over a period of 30minutes. The mixture was stirred for 2 hours to complete polymerization,and the obtained emulsion was cooled, salted out by aluminum chloride,filtered, washed, and dried to obtain a polymer.

Examples 2 through 8

In the same reaction vessel as used in Example 1, polymers were obtainedunder the same polymerization conditions as adopted in Example 1, usingthe starting materials shown in Table 1.

Comparative Example 1 through 11

In Comparative Example, the lubricant of the present invention was notadded to a vinyl chloride resin but a stabilizer was added. InComparative Examples 2 through 11, polymers were prepared in the samemanner as described in Example 1 except that the conditions were outsidethe scope of the present invention, as shown in Table 1.

As apparent from Table 1, in Comparative Examples 2, 3 and 6,agglomeration (blocking) of the polymers occurred at post treatmentssuch as coagulation, dehydration and drying, and the productivity waslow.

Preparation and Evaluation of Vinyl Chloride Resin Composition

Vinyl chloride resin compositions of the present invention (Examples)and the Comparative Examples were prepared by mixing 100 parts of avinyl chloride resin having an average polymerization degree of 700, 1.5parts of dibutyl-tin mercaptide, 1 part of an epoxy type aid, 0.5 partof dibutyl-tin maleate, and 3 parts of the polymer sample obtained aboveby a Henschel mixer. The vinyl chloride resin compositions wereevaluated by the evaluation methods described below. The results areshown in Table 1.

(1) Stickiness

By using 6-inch rolls, 100 g of the sample was kneaded at rolltemperature of 205° C. and 200° C. with a roll spacing of 1 mm, and thetime required for bringing about the state wherein the sheet adhered tothe roll surface and was not peeled therefrom was measured. The longerthis time, the better the durability of the lubricating effect at a hightemperature.

(2) Gelling characteristics (lubricating properties and adherence) byBrabender Plasti-Corder

Tmax:

The time (minutes) required for arriving at a maximum torque (kg.m) atthe measurement using Brabender Plasti-Corder was determined. Thesmaller the Tmax value, the more quickly melted the resin and the poorerthe lubricating properties.

Mmax:

The maximum torque (kg.m) was determined at the above measurement. Thelarger the Mmax value, the higher the melt viscosity of the resin andthe poorer the lubricating properties.

Adherence:

Kneading was carried out for 15 minutes at a temperature of 180° C. anda rotation number of 30 rpm with a packed amount of 50 g, and theadherence of the resin to the vessel wall was examined and evaluatedaccording to the standard of from 1 to 10. A smaller value indicatesbetter lubricating properties.

(3) Flowability

By using a single-screw extruder having a screw diameter of 25 mm, theamount of the sample extruded for 3 minutes was measured (extrudertemperatures: C₁ =160° C., C₂ =170° C., C₃ =180° C. die=180° C.;rotation number=40 rpm). The larger the extruded amount, the better theflowability.

(4) Transparency

By using 6-inch rollers, 100 g of the sample was kneaded at rolltemperatures of 185° C. and 180° C. for 5 minutes with a roll spacing of1 mm, and the kneaded sample was compressed (40 kg) at 185° C. to form atest piece having thickness of 2 mm. By using an integration sphere typehaze meter, the haze value and total luminous transmittance weremeasured according to the method of JIS K-6714.

Examples 9 through 14 and Comparative Examples 12 through 17

By using a Henschel mixer, 100 parts of a thermoplastic resin shownbelow, in which 3 parts of the polymer obtained in Example 1 wasincorporated or was not incorporated, was mixed and was then extruded byusing a single-screw extruder having a screw diameter of 25 mm. Theextrusion temperatures were changed according to the kinds of thethermoplastic resins used, as shown below. With respect to each resin,the extruded amount was measured to determine the flowability, and theextrusion pressure was measured at the time of extrusion. The resultsare shown in Table 2.

(1) ABS resin (Diapet® ABS#3001 supplied by Mitsubishi Rayon) (moldingtemperatures: C₁ =180° C., C₂ =200° C., C₃ =200° C., head=200° Cdie=200° C.)

(2) Styrene resin (Styrol NF-20 supplied by Idemitsu Petrochemical)(molding temperatures: C₁ =160° C., C₂ =180° C., C₃ =200° C., head=200°C., die=210° C.)

(3) Polycarbonate resin (Novalex 7022 supplied by Mitsubishi Chem. Ind.)(molding temperatures: C₁ =230° C., C₂ =260° C., C₃ =270° C., head=270°C., die=80° C.)

(4) Polyethylene resin (Hizex 7000F supplied by Mitsui Petrochemical)(molding temperatures: C₁ =150° C., C₂ =165° C., C₃ =175° C., head=175°C., die=75° C.)

(5) Polyester resin (Dianite® A-200 supplied by Mitsubishi Rayon)(molding temperatures: C₁ =280° C., C₂ =280° C., C₃ =280° C., head=260°C., die=260° C.)

(6) Vinyl chloride resin (Ryonichi PVC supplied by Ryonichi KK:polymerization degree=700) (molding temperature: C₁ =160° C., C₂ =170°C., C₃ =180° C. head=175° C., die=180° C.)

As apparent from the results shown in Table 2, if the polymer obtainedin Example 1 was incorporated in a thermoplastic resin, the extrusionpressure was reduced and the extruded amount was increased, regardlessof the kind of thermoplastic resin, and an excellent lubricating effectwas attained.

                                      TABLE 1                                     __________________________________________________________________________           Composition of three-stage polymer         Flowa-                                                     Three-             bility                                                                            Transparency                   First stage A                                                                         Second stage B                                                                        Third stage C                                                                         stage              Ex- Total                          MMA     St/EA   MMA     poly-   Gelling characteristics                                                                  truded                                                                            luminous                       (parts) (parts) (parts) mer Stick-  Mmax                                                                              Ad-                                                                              amount                                                                            trans-                                                                             Haze                      n-OSH                                                                             ηsp/C                                                                         n-OSH                                                                             ηsp/C                                                                         n-OSH                                                                             ηsp/C                                                                         ηsp/C                                                                         ness                                                                              Tmax                                                                              (kg ·                                                                    her-                                                                             (g/3                                                                              mittance                                                                           value              Sample (parts)                                                                           (dl/g)                                                                            (parts)                                                                           (dl/g)                                                                            (parts)                                                                           (dl/g)                                                                            (dl/g)                                                                            (min)                                                                             (min)                                                                             m)  ence                                                                             min)                                                                              (%)  (%)                __________________________________________________________________________    Example 1                                                                            30  1.0 36/24                                                                             0.6 10  1.0 0.7 43  5.0 4.2 1.0                                                                              150 85.7 5.8                       0.1     0.5     0.05                                                   Example 2                                                                            30  1.0 (St/                                                                              0.5 10  1.0 0.6 50  5.2 4.0 1.0                                                                              155 85.0 6.2                       0.1     BA)     0.05                                                                  36/24                                                                         0.5                                                            Example 3                                                                            15  1.0 36/24                                                                             0.6 25  1.0 0.7 40  4.5 4.3 1.2                                                                              145 85.9 5.7                       0.1     0.5     0.05                                                   Example 4                                                                            30  1.0 30/20                                                                             0.6 20  1.0 0.8 38  4.2 4.5 1.6                                                                              143 85.5 6.0                       0.1     0.5     0.05                                                   Example 5                                                                            30  1.4 30/20                                                                             0.6 20  1.5 1.3 35  4.0 4.8 2.5                                                                              140 85.6 5.9                       0.08    0.5     0.03                                                   Example 6                                                                            30  1.0 30/20                                                                             0.9 20  1.0 0.9 30  3.7 5.0 3.0                                                                              137 85.7 6.0                       0.1     0.4     0.05                                                   Example 7                                                                            20  1.0 42/18                                                                             0.7 20  1.0 0.8 30  3.8 5.0 2.8                                                                              138 86.2 5.6                       0.1     0.5     0.05                                                   Example 8                                                                            20  1.0 18/42                                                                             0.5 20  1.0 0.6 48  5.2 4.0 1.0                                                                              155 84.0 6.8                       0.1     0.5     0.05                                                   Comparative                                                                          --  --  --  --  --  --  --  3.0 2.0 5.0 10.0                                                                             120 86.0 5.7                Example 1                                                                     Comparative                                                                          --  --  50/50                                                                             0.6 --  --  0.6 --  --  --  -- --  --   --                 Example 2*.sup.1                                                                             0.5                                                            Comparative                                                                          40  3.0 30/30                                                                             0.6 --  --  1.8 40  4.2 4.5 3.0                                                                              120 85.0 5.7                Example 3*.sup.2                                                                     0.007   0.5                                                            Comparative                                                                          --  --  30/30                                                                             0.6 40  3.0 1.8 15  3.0 5.5 5.0                                                                              125 86.0 6.2                Example 4      0.5     0.03                                                   Comparative                                                                          40  1.0 12/8                                                                              0.6 40  1.0 0.8 7.0 3.2 5.6 7.0                                                                              124 86.2 6.0                Example 5                                                                            0.1     0.5     0.05                                                   Comparative                                                                          16  1.0 48/32                                                                             0.6 4   1.0 0.7 35  *4.8                                                                              4.0 1.2                                                                              138 85.4 6.3                Example 6*.sup.1                                                                     0.1     0.5     0.05                                                   Comparative                                                                          35  2.2 30/20                                                                             0.6 15  1.0 1.8 20  2.5 5.2 5.0                                                                              125 85.8 5.9                Example 7                                                                            0.05    0.5     0.05                                                   Comparative                                                                          35  1.0 30/20                                                                             2.8 15  2.5 1.6 18  2.4 5.2 5.0                                                                              124 85.7 6.0                Example 8                                                                            0.1     0.5     0.015                                                  Comparative                                                                          35  1.0 30/20                                                                             2.8 15  1.0 2.0 12  1.8 5.4 8.0                                                                              120 86.0 5.8                Example 9                                                                            0.1     0.03    0.05                                                   Comparative                                                                          30  1.0 45/5                                                                              0.7 20  1.0 0.9 7.0 3.2 5.5 8.0                                                                              121 86.2 5.6                Example 10                                                                           0.1     0.5     0.05                                                   Comparative                                                                          30  1.0  5/45                                                                             0.5 20  1.0 0.7 40  5.0 4.2 1.2                                                                              148 80.0 13.0               Example 11                                                                           0.1     0.5     0.05                                                   __________________________________________________________________________     *.sup.1 The polymer was blocked during coagulation and could not be           powdered.                                                                     *.sup.2 The polymer was blocked during coagulation and was difficult to       powder.                                                                       MMA: methyl methacrylate                                                      BA: butyl acrylate                                                            EA: ethyl acrylate                                                            St: styrene                                                                   n-OSH: noctyl mercaptan                                                  

                  TABLE 2                                                         ______________________________________                                                                  Pressure                                                   Kind of  Addition of                                                                             at       Extruded                                          thermoplastic                                                                          polymer of                                                                              extrusion                                                                              amount                                            resin    Example 1 (kg/m)   (g/10 min)                                 ______________________________________                                        Example 9                                                                              ABS resin  Added     32     370                                      Comparative                                                                   Example 12          Not added 40     300                                      Example 10                                                                             Styrene resin                                                                            Added     40     380                                      Comparative                                                                   Example 13          Not added 45     340                                      Example 11                                                                             Polycarbonate                                                                            Added     35     320                                      Comparative                                                                            resin                                                                Example 14          Not added 42     240                                      Example 12                                                                             Polyethylene                                                                             Added     30     390                                      Comparative                                                                            resin                                                                Example 15          Not added 35     350                                      Example 13                                                                             Polyester  Added     36     360                                      Comparative                                                                            resin                                                                Example 14          Not added 42     290                                      Example 14                                                                             Vinyl chloride                                                                           Added     35     350                                      Comparative                                                                            resin                                                                Example 15          Not added 40     320                                      ______________________________________                                    

We claim:
 1. A lubricant for a thermoplastic resin, which comprises athree-stage polymer obtained by (A) polymerizing 80 to 100% by weight ofmethyl methacrylate and 0 to 20% by weight of other monomercopolymerizable therewith so that the reduced viscosity ηsp/C (asdetermined at 25° C. with respect to a solution of 0.1 g of the polymerin 100 ml of chloroform) of the obtained polymer is not higher than 1.4dl/g, (B) polymerizing 30 to 70 parts by weight of a monomer mixturecomprising 20 to 80% by weight of an acrylic acid ester and 80 to 20% byweight of styrene or a substituted styrene in the presence of 10 to 60parts by weight of the polymer obtained at stage (A), so that thereduced viscosity ηsp/C of the copolymer obtained by polymerizing themonomer mixture in the absence of the polymer obtained at stage (A) isnot higher than 1 dl/g, and (C) polymerizing 5 to 30 parts by weight ofa monomer or monomer mixture comprising 50 to 100% by weight of amethacrylic acid ester and 0 to 50% by weight of other monomercopolymerizable therewith in the presence of the two-stage polymerformed through stages (A) and (B), so that the total amount of thecomponents used at the stages (A), (B) and (C) is 100 parts by weightand the reduced viscosity ηsp/C of the polymer obtained by polymerizingthe monomer or monomer mixture in the absence of the two-stage polymerformed through stages (A) and (B) is not higher than 1.5 dl/g.
 2. Thelubricant according to claim 1, wherein the monomer copolymerized withmethyl methacrylate in stage (A) is an ethylenically unsaturated monomerselected from the group consisting of unsaturated nitriles, vinylesters, acrylic acid esters, and methacrylic acid esters other thanmethyl methacrylate.
 3. The lubricant according to claim 1, wherein theamount of the polymer prepared in stage (A) and used in stage (B) is 10to 50 parts by weight per 100 parts by weight of the total amount of thepolymers prepared in stages (A), (B) and (C).
 4. The lubricant accordingto claim 1, wherein the acrylic acid ester is selected from the groupconsisting of ethyl acrylate, butyl acrylate, benzyl acrylate,2-ethylhexyl acrylate, cyclohexyl acrylate and phenyl acrylate, and thesubstituted styrene is α-methyl styrene.
 5. The lubricant according toclaim 1, wherein the amount of the monomer mixture used in stage (B) is35 to 70 parts by weight per 100 parts by weight of the total amount ofthe polymers prepared in stages (A), (B) and (C).
 6. The lubricantaccording to claim 1, wherein the methacrylic acid ester used in stage(C) is selected from the group consisting of methyl methacrylate, ethylmethacrylate, butyl methacrylate, benzyl methacrylate, 2-ethylhexylmethacrylate, cyclohexyl methacrylate, and phenyl methacrylate, and themonomer copolymerized with the methacrylic acid ester in stage (C) is anethylenically unsaturated monomer selected from the group consisting ofunsaturated nitriles, vinyl esters and acrylic acid esters.
 7. Thelubricant according to claim 1, wherein the amount of the monomer ormonomer mixture used in stage (C) is 5 to 25 parts by weight per 100parts by weight of the total amount of the polymers prepared in stages(A), (B) and (C).
 8. The lubricant according to claim 1, wherein thethree-stage polymer has a reduced viscosity ηsp/C of not higher than 1.5dl/g.